1. Field of the Invention
This invention relates to high-fidelity audio reproduction generally, and more specifically to the origination, transmission, recording, and reproduction of digital audio, especially encoded or compressed multi-channel audio signals.
2. Description of the Related Art
Digital audio recording, transmission, and reproduction has exploited a number of media, such as standard definition DVD, high definition optical media (for example “Blu-ray discs”) or magnetic storage (hard disk) to record or transmit audio and/or video information to the listener. More ephemeral transmission channels such as radio, microwave, fiber optics, or cabled networks are also used to transmit and receive digital audio. The increasing bandwidth available for audio and video transmission has led to the widespread adoption of various multi-channel, compressed audio formats. One such popular format is described in U.S. Pat. Nos. 5,974,380, 5,978,762, and 6,487,535 assigned to DTS, Inc. (widely available under the trademark, “DTS” surround sound).
Much of the audio content distributed to consumers for home viewing corresponds to theatrically released cinema features. The soundtracks are typically mixed with a view toward cinema presentation, in sizable theater environments. Such a soundtrack typically assumes that the listeners (seated in a theater) may be close to one or more speakers, but far from others. The dialog is typically restricted to the center front channel. Left/right and surround imaging are constrained both by the assumed seating arrangements and by the size of the theater. In short, the theatrical soundtrack consists of a mix that is best suited to reproduction in a large theater.
On the other hand, the home-listener is typically seated in a small room with higher quality surround sound speakers arranged to better permit a convincing spatial sonic image. The home theater is small, with a short reverberation time. While it is possible to release different mixes for home and for cinema listening, this is rarely done (possibly for economic reasons). For legacy content, it is typically not possible because original multi-track “stems” (original, unmixed sound files) may not be available (or because the rights are difficult to obtain). The sound engineer who mixes with a view toward both large and small rooms must necessarily make compromises. The introduction of reverberant or diffuse sound into a soundtrack is particularly problematic due to the differences in the reverberation characteristics of the various playback spaces.
This situation yields a less than optimal acoustic experience for the home-theater listener, even the listener who has invested in an expensive, surround-sound system.
Baumgarte et al., in U.S. Pat. No. 7,583,805, propose a system for stereo and multi-channel synthesis of audio signals based on inter-channel correlation cues for parametric coding. Their system generates diffuse sound which is derived from a transmitted combined (sum) signal. Their system is apparently intended for low bit-rate applications such as teleconferencing. The aforementioned patent discloses use of time-to-frequency transform techniques, filters, and reverberation to generate simulated diffuse signals in a frequency domain representation. The disclosed techniques do not give the mixing engineer artistic control, and are suitable to synthesize only a limited range of simulated reverberant signals, based on the interchannel coherence measured during recording. The “diffuse” signals disclosed are based on analytic measurements of an audio signal rather than the appropriate kind of “diffusion” or “decorrelation” that the human ear will resolve naturally. The reverberation techniques disclosed in Baumgarte's patent are also rather computationally demanding and are therefore inefficient in more practical implementations.